net: sfp: add support for Clause 45 PHYs

Some SFP+ modules have a Clause 45 PHY onboard, which is accessible via
the normal I2C address.  Detect 10G BASE-T PHYs which may have an
accessible PHY and probe for it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>